Twenty-seven young (<50 years old) patients with spontaneous carotid artery dissection in 11 cases and carotid atherosclerosis in 16 cases were evaluated to determine the apolipoprotein E polymorphism. At the DNA analysis the epsilon3/epsilon3 genotype was observed in all patients with dissection, in 13 of 16 (81.2%) patients with atherosclerosis and in 27 of 30 (90%) controls. Three of 16 (18.8%) patients with atherosclerosis and 3 of 30 (10%) controls presented with the epsilon4/epsilon3 genotype, and this difference was not statistically significant. Moreover, observation of the epsilon4/epsilon3 genotype was not significantly higher in patients with atherosclerosis compared with those with dissection. No homozygote epsilon4/epsilon4, epsilon2/epsilon2 or heterozygote epsilon2 genotype was observed. No correlation was found between the presence of the epsilon4/epsilon3 genotype and vascular risk factors. Therefore, the epsilon4 allele seems to be involved in carotid premature atherosclerosis development whereas it may appear to be protective for artery dissection occurrence. A larger sample size is needed to support this suggestion.